1. Field of the Invention
The invention relates to the synthesis of mercaptans derived from corresponding catechols via the thionocarbamate-thiolcarbamate rearrangement by replacement of one of the hydroxyls by mercaptan. The invention encompasses catechol (1,2-dihydroxybenzene) and multi-ring aromatic compounds containing the catechol function, i.e., two adjacent hydroxyls on any ring.
2. The Prior Art
The chemical preference of divalent sulfur for forming single rather than double bonds to carbon is well known. It is not surprising, therefore, that the reactivity of thiocarbonyls is quite different from that of their oxygen analogues. The thermal transformation of aryl thionocarbonates into the corresponding thiolcarbonates has been called the Schonberg rearrangement after Alexander Schonberg, who first observed it in 1929. [A. Schonberg and L. Varga, Chem. Ber. 63, 178 (1930); A. Shonberg, L. Varga, and W. Paul, Ann. Chem., 483, 107, (1930)]: ##STR9##
Schonberg summarized his observations by 3 empirical rules for the rearrangement:
(1) The rearrangement will proceed only in acyclic systems. Cyclic thionocarbonates, such as ##STR10## will not rearrange.
(2) In all cases observed, the rearrangement proceeded from a higher melting to a lower melting compound.
(3) The carbon atom on which a bond to oxygen is broken in the rearrangement is the same carbon atom to which a bond to sulfur is formed. That is to say, there is no secondary rearrangement. The substitution pattern on the aryl portion of the molecule is not altered.
Schonberg pointed out the significance of the thionocarbonate rearrangement as providing a means of converting phenols into their thio analogues. He did not, however, actually reduce his disulfides to thiophenols.
Tarbell and coworkers [H. R. Al-Kazimi, D. S. Tarbell, and D. Plant, J. Amer. Chem. Soc., 77, 2479 (1955)] pursued the investigation of the thionocarbonate rearrangement, but not until 25 years after Schonberg's work. Their study involved the thermal rearrangement of 12 thionocarbonates, of which 6 were unsymmetrical (X .noteq. Y): ##STR11## Tarbell hydrolyzed his thiolcarbonates and then oxidized the resulting substituted thiophenols to disulfides, thus extending the scope of the general synthetic method suggested by Schonberg.
Kwart and Evans [H. Kwart and E. R. Evans, J. Org. Chem., 31, 410 (1966)] suggested that the thionocarbonate rearrangement ought to proceed in the vapor phase. The advantage of a vapor phase reaction is that by avoiding prolonged heating in a polar medium one might also avoid side reactions and decompositions which consume both products and reactants. Kwart and Evans investigated the rearrangement of bis-(0-2-allylphenyl) thionocarbonate: ##STR12## The results supported their original contentions about the thionocarbonate rearrangement. More important was their application of the method and principle to thionocarbamates. They were able to predict and demonstrate a thermal rearrangement of thionocarbamates to thiolcarbamates, in analogy to the thionocarbonate rearrangement.
The superiority of the thionocarbamate rearrangement to the Schonberg method for synthesis of thiophenols was immediately recognized and pursued by Newman and Karnes [M. S. Newman and H. A. Karnes, J. Org. Chem., 31, 3980 (1966)]. Even at maximum efficiency, rearrangement of a thionocarbonate can only provide 1 mole of thiophenol for every 2 moles of phenol in the starting material. On the other hand, the conversion of phenols to thiophenols via a thionocarbamate rearrangement is theoretically 100 percent efficient.
Newman and Karnes prepared over 30 substituted O-aryl dialkyl-thionocarbamates by base catalyzed addition of substituted phenols to N,N-dialkylthiocarbamyl chlorides: ##STR13## In most cases, thermal rearrangement of these products to the corresponding thiolcarbamates required lower temperatures and shorter reaction times, and gave better yields than the corresponding thionocarbonate rearrangements by Tarbell. Newman's thiolcarbamates were hydrolyzed in good yield to substituted thiophenols. There were significant exceptions to the usually successful rearrangements of thionocarbamates by Newman, however. More significant is Newman's report that 4 compounds, 3 of them derivatives of catechol, failed to rearrange even at high temperature, but decomposed to tar: ##STR14## The apparent exclusion of catechols from conversion by this method to mono-thio catechols excludes a very large family of natural products bearing the catechol function, as well as many synthetically useful catechol derivatives, 2-methoxyphenyl thionocarbamate, a catechol der vative, rearranged, but only at very high temperature (290.degree. C). The scope of Newman and Karnes' investigation firmly establishes the thionocarbamate-thiolcarbamate rearrangement as the method of choice for conversion of phenols to thiophenols. Applicants pursued this route to synthesize thio derivatives of catechols in spite of the previous failures.
(A more comprehensive review of the prior art can be found in "Extrusions and Sulfur-Oxygen Rearrangements of Catechol Derivatives" (1976), the doctoral thesis of Joseph B. Hanson, one of the applicants herein, available from Xerox University Microfilms, Ann Arbor, Mich., and incorporated herein by reference.)